The hypothesis that slow wave sleep (SWS) is a thermoregulatory process counteracting hypothermia and conserving energy expenditure will be tested by determining whether putative manipulations of human brain temperature (Tbr), by heating and cooling the face, influence the propensity for SWS (stages 3+4). Since Tbr cannot be directly measured in human subjects, tympanic temperature (Tty) will serve as a practical index of Tbr. Rectal temperature (Tre) and skin temperature of the foot (Tft) will also be recorded. In each of two experiments, nine Ss will be studied, each for eight nights. Following two adaptation nights to the apparatus and laboratory, they will be subjected to two nights in each of three condition : baseline, facial heating and facial cooling. A Latin square design will counterbalance the order of conditions across Ss. In one experiment therma manipulations will begin 30 min prior to lights out and continue throughout the night. In the other they will be restricted to the last 3 hr of the night--a period when SWS is normally absent. Electrophysiological records (EEG, EOG and EMG) will be visually scored for stages of sleep and wakefulness, and EEG slow wave activity will be analyzed with a computer using fast Fourier spectral analysis. Hourly and nightly mean values of Tty, Tre, Tft, SWA and sleep stage variables will be compared between conditions in each experiment by paired differences t-tests. Pearson product-moment correlations will be computed hourly SWA and Tty, Tre, Tft for each S across all nights in each experimen , and an average r across Ss after a Z transformation. Similar correlations will be computed between SWS (stages 3 + 4) and Tb measures, and between other sleep stages and Tb measures. The experiments should elucidate the biological functions of sleep, by testing whether the propensity for SWS is related to Tbr. They may also le d to clinical applications in thermal treatment of sleep disorders--especiall insomnia. Low amounts of stage 4 SWS and minimal decreases in rectal temperature during sleep are characteristic of depressed patients compared with control subjects. Therefore, alterations in heat balance might also prove effective in the treatment of depression.